1. Field of the Invention
The present invention generally relates to a battery-powered electronic device. More particularly, the invention relates to a technique for providing power to components in an electronic device in such a way that the devices can determine when an overload condition is occurring.
2. Background of the Invention
Most every computer system with a rechargeable battery uses an external AC to DC converter (sometimes called an “adapter”) which converts the AC line voltage to a lower DC voltage. An example is shown in FIG. 1 in which an AC adapter 10 provides power for a computer 12 and a battery subsystem 14. The battery subsystem includes a battery charger and a battery. The current sense circuit 16 will be discussed below.
Due to its size and shape, the adapter has often been referred to as the “brick.” The brick is usually external to the computer shell and is often an awkward part of the system to store and carry. While using AC power, the brick supplies power both for the normal operation of the computer and also for recharging the battery.
Typical AC/DC converters are provided with an input of 100 to 240 VAC and generate an output voltage of 18 VDC with a total power output capacity of 50 to 70 watts. The size (i.e., power capacity) of the AC adapter is normally established by estimating a reasonable “power budget” for the CPU. The power budget is a total of the maximum power consumption of the computer's internal devices (the CPU, core chipset, LCD panel, hard drive, etc.) plus some allocation for externally powered devices (e.g., USB, PS/2, or external storage).
Older notebook computers with small LCD screens and low power processors typically consumed a maximum of 10 or 15 watts while operational. Today's notebooks, however, with 15″ high resolution screen, multiple internal storage drives, and gigahertz processors can easily consume 50 to 60 watts of power. Moreover, performance requirements have demanded bigger AC/DC adapters which are designed to be sufficient for the worst case power consumption of the system.
While the power demands for portable computers continuously increases, the pressure to make the system “mobile” places pressure on the system designer to make the AC/DC brick as small as possible. Ergonomics discourages large AC/DC adapters which dissipate proportionately more heat. Further, cost pressures prohibit the use of more powerful or more efficient AC/DC bricks. Yet, at the same time, it is desirable for the computer to be able to charge the battery as quickly as possible. In sum, many consumers desire portables that have high performance (e.g., fast CPUs, bright displays, etc.), recharge batteries very quickly, are lightweight and small, inexpensive, and do not become hot to the touch.
To date, the concession to AC/DC size has been to “throttle” battery charge when the rest of the system is under full loading. In many older systems, the “power budget” and AC/DC adapter size were calculated by estimating the consumption of the computer, and then allocating an additional amount of power for recharging the battery. Today, the one common concession towards power budget allocation is that power for the recharge of the battery itself is not included in the power budget on which the adapter is designed. This means that most adapters today are rated to provide sufficient power for the computer at full load, but not for charging the battery with the computer at full load. Thus, notebooks today measure the core system power consumption and then allocate the remaining AC/DC power (if there is any remaining power) to charge the battery.
Such conventional systems include, as shown in FIG. 1, a current sense circuit 16 that receives the output voltage from the adapter and passes that voltage on to the computer 12 and battery subsystem 14. The current sense circuit generally includes a low resistance current sense resistor (e.g., 50 milliohms) in series with the power flow to the computer and battery subsystem, as well as an amplifier that amplifies the voltage across the sense resistor. The amplifier circuit is designed so as to assert an output signal 18 when the current out of the adapter exceeds a certain threshold. Conventional AC adapters 10 are constant voltage (“CV”) adapters which means their output voltage is regulated to a predetermined value (e.g., 18 VDC) as illustrated graphically in FIG. 2. Because the output voltage is constant, the output current can be used to determine output power. Thus, the output signal 18 from the current sense circuit 16 is asserted, in effect, when the power draw on the adapter by loads 12 (the computer) and 14 (the battery subsystem) nears or exceeds the output power rating of the adapter. In FIG. 2, the over power condition occurs when the current output of the adapter is above Imax.
The current sense circuit output signal 18 typically is provided to the battery subsystem 14 to alert the battery subsystem that the adapter 10 cannot keep up with the power demands of the computer 12 and battery subsystem 14 combined. The battery subsystem 14 uses signal 18 to “throttle” back on battery charge current. Throttling back charge current means to reduce the charge current into the battery. Throttling back charge current results in a lower power draw on the AC adapter thereby alleviating the over power condition. The battery subsystem 14 may even cease battery charging altogether if necessary to protect the adapter 10. By throttling back battery charging, the adapter's output current will not exceed Imax.
Although a generally satisfactory implementation, the current sense circuit 16, which is part of the computer, is not a trivially simple circuit to design. For instance, the amplifier in the circuit may need to be operated rail-to-rail which complicates the amplifier design. Further, voltage level shifting may be required also complicating the implementation. These contribute to error in the resulting current sense output. Accordingly, an alternative system is needed which avoids the problems noted above with the current sense circuit 16.